Hitherto, when an output signal (high frequency signal) amplified by a preamplifier is transmitted through a channel, there is a problem in that a high frequency component of the output signal is attenuated during the transmission, and the waveform thereof deteriorates due to a dull rising/falling edge. In order to avoid such waveform deterioration, an emphasis function needs to be implemented so as to increase the amplitude of a signal only in the vicinity of a rising/falling edge thereof, to thereby compensate for waveform deterioration in a high frequency band in a signal output line.
Then, there is an emphasis circuit that includes a delay circuit, a main output driver, an emphasis output driver, and an output driver in order to implement the emphasis function (see, for example, Patent Literature 1). In this emphasis circuit, an input differential signal is branched into two signals and added with a predetermined delay amount by the delay circuit inserted in one signal propagation line. The signals are input to the main output driver in the other signal propagation line in which the delay circuit is not inserted, and are input to the emphasis output driver in the signal propagation line in which the delay circuit is inserted. Then, a positive phase output of the main output driver and a negative phase output of the emphasis output driver are multiplexed, and a common output load resistor is driven by the output driver.
Further, there is another emphasis circuit that includes a flat-response amplifier block (baseband amplifier), a band pass filter (BPF), a peak-response amplifier block (peaking amplifier), and an adder circuit (linear amplifier) in order to implement the emphasis function. In this emphasis circuit, an input differential signal is branched into two signals, and the output amplitude of the baseband is determined by the flat-response amplifier block. Further, due to the peak-response amplifier block having the band pass filter inserted at an input section thereof, the gain peak can be provided in such a high frequency band (for example, 6 GHz to 8 GHz) that has a large loss in an output line. Then, an output from the flat-response amplifier block and an output from the peak-response amplifier block are multiplexed, and an output load resistor is driven by the adder circuit.
Further, there is another method for implementing a peaking amplifier by adding the inductance to a load resistor without inserting the band pass filter (BPF) at the input section, although this method is applied to an equalizer function on the receiver side rather than an emphasis function on the transmitter side (see, for example, Non Patent Literature 1).